1. Field of the Invention
This invention is related to circuit interrupters and more particularly to a current limiting circuit interrupter wherein a movable contact arm is disposed within the slot of a magnetic drive device and a pair of separable contacts are disposed within the opening of an arc driving yoke.
2. Description of the Prior Art
It is common in the prior art to provide for current limiting during circuit fault conditions. A common method of providing current limiting is through the use of current limiting fuses in combination with a standard stored energy type circuit interrupter as disclosed in U.S. Pat. No. 3,077,525 to Dyer. U.S. Pat. No. 3,815,059 to L. A. Spoelmen discloses a circuit interrupter in series with an electromechanical current limiting device which utilizes the force generated by the overload current to drive the movable contact arm open. In U.S. Pat. No. 3,815,059 a current limiting interrupter was provided with a movable contact arm which is pivoted around one end and which moves into the slot of a magnetic drive device during circuit interruption.
When dealing with current limiting circuit interrupters the term I.sup.2 t is often utilized to describe the effectiveness of a particular device. The meaning of I.sup.2 t can best be understood by considering an electrical source feeding a simple resistive load through the circuit interrupter. The total energy seen by the load is given by: current (I) flowing through the load times the voltage drop (IR) across the load times the time (t) current flows. This can be expressed mathematically by; energy dissipated equals RI.sup.2 t. During normal operation this energy RI.sup.2 t is easily dissipated by the system. When a fault occurs, however, current (I) can become very large in value. For example, with the present day power supplies potential fault currents up to 100,000 amperes can occur. During fault conditions the value of the load, R, will remain approximately the same but the total energy input into the system will be very large. In a practical system if the load is to be protected the fault time should be limited to as small a value as possible. A reduction of the time of fault current flow, t, will give a further reduction in the energy input into the load. This means that in any current limiting device the total I.sup.2 t, i.e., (the integrated fault current).sup.2 x (time it flows) is the important parameter to measure. The smaller the I.sup.2 t value, the better the performance of the current limiting device.
In the circuit interrupter art, it has long been recognized that it is of a distinct advantage to provide fast interruption of an established arc. It is well known by those skilled in the art that it is desirable to effect a rapid extinction of the arc as quick as possible inasmuch as the fault current flow through connected electrical equipment will damage the equipment unless the fault current is limited. Due to heating, voltage surges, and other harmful effects it is desirable to effect extinction of the fault current as soon as possible after initiation.
In accordance with the teaching of the present invention, a pair of spaced relatively stationary contacts are bridged by a movable bridging contact arm carrying two movable contacts therewith which cooperate with the two spaced stationary contacts to establish two serially related arcs during circuit interruption. Each set of mating contacts is disposed within the yoke of a magnetic arc driving structure which effects the rapid outward movement of the arc. A set of arc-extinguishing plates extends within the yoke in proximity to each set of mating contacts. The magnetic field set up by the current within the circuit interrupter in the yoke affects a rapid lateral outward movement of the arc into the arc-extinguishing plate structure.
Current limiting can be achieved by establishing a rapid rise of arc voltage. Arc voltage can be increased rapidly by separating the contacts in a short period of time and/or by elongating and rapidly driving the arc formed into a set of spaced arc-extinguishing plates. In the disclosed invention, a magnetic drive or linear slot motor is used to drive the contact arm which bridges the two pairs of mating contacts, thereby affecting rapid contact separation. A magnetic arc drive or yoke is used at each pair of contacts to elongate and drive the arc into T-shaped spaced arc-extinguishing plates. These nonmagnetic plates can be formed of metal, metal sprayed with insulation, or insulation. The combination of these two methods of obtaining a rapid rise in arc voltage results in a current limiting circuit breaker providing excellent current limitation. The magnetic arc drive rapidly moves any arc formed off of the contacts which allows the use of silver cadmium oxide, AgCdO, contacts which lower the temperature rise at the terminal. In prior art circuit interrupters silver tungsten contacts are normally required for high current interruption. In one embodiment of the invention arc horns are formed extending outward from the contacts towards the arc-extinguishing plates.
It is an object of this invention to teach a circuit interrupter having a movable contact arm which is rapidly drawn into the slot of a magnetic drive device during circuit interruption and having a yoke formed of a magnetizable material disposed around the contacts which are separated during circuit interruption.